Development for practical use of a semiconductor laser element emitting green light is intensively ongoing. The use can include, for example, an image display device or a pointer which has the semiconductor laser element as a light source. However, a true green semiconductor laser element that emits light with a wavelength of around 530 nm has not been put into practical use, but a solid laser which is expensive and has low efficiency has been used up until now. However, in recent years, a semiconductor laser element which is configured to have a GaN-based compound semiconductor and emits true green light is reported, and the practical use of the semiconductor laser element is expected.
In a light-emitting element made of a laminated structure of GaN-based compound semiconductor layers, a polar surface such as a (0001) c surface of the GaN substrate is often used as a main surface of a substrate for manufacturing the light-emitting element. However, when a light-emitting element in which, for example, an n-type GaN layer, an active layer made of InGaN, and a p-type GaN layer are laminated is manufactured by using a (0001) c surface of the GaN substrate as a main surface, a lattice constant of an InGaN crystal is slightly larger than a lattice constant of a GaN crystal. Accordingly, when the n-type GaN layer, the active layer, and the p-type GaN layer whose respective top faces are C surfaces are laminated, piezo spontaneous polarization occurs in a thickness direction of the active layer due to compression pressure applied on the active layer. As a result, a shift in a light emission wavelength from the light-emitting element, a decrease in light emission efficiency or luminance saturation occurs.
In order to avoid such a problem, a technology of using a semi-polar surface such as a (20-21) surface as a main surface of the GaN substrate for manufacturing a light-emitting element is well-known from, for example, PTL 1. Specifically, a semiconductor laser element disclosed in the patent publication includes a gallium nitride substrate which has the {20-21} surface as a first main surface, a nitride semiconductor thick film provided in contact with the first main surface of the gallium nitride substrate, and a nitride semiconductor laser element layer which is provided on the nitride semiconductor thick film, in which the nitride semiconductor laser element layer includes an n-type nitride semiconductor layer, a light emission layer which is made of a nitride semiconductor, and a p-type nitride semiconductor layer, and the nitride semiconductor thick film has a {−1017} surface which is within a range of making an angle from 89.95 degrees to 90.05 degrees with the {20-21} surface that is a main surface of the nitride semiconductor thick film.
Here, the nitride semiconductor thick film is provided to ease lattice distortion. In addition, since the nitride semiconductor thick film has a {−1017} surface within a range of making an angle alpha from 89.95 degrees to 90.05 degrees with the {20-21} surface which is a main surface of the nitride semiconductor thick film, it is possible to easily form a good resonator end surface on a nitride semiconductor laser element layer on the nitride semiconductor thick film by a cleavage. Therefore, it is said that it is possible to realize a low threshold current even when using the GaN substrate in which the {20-21} surface that is a semi-polar surface is set to a first main surface. Moreover, for example, marks of crystal surfaces in a hexagonal system illustrated below, are written as {hk-il} surface and {h-kil} surface for convenience in the present specification.
{h k ī l} surface
{h k i l} surface